As the user demands higher and higher working speed of a computer, the electric power needed by CPU and the peripheral devices in a computer system grows responsively; therefore, the power supply of the computer system has to provide more power output. To secure users' safety and health, there is a limitation on the power output by a power supply. Currently, the maximum power output (max VA) for a power supply is generally 240VA; for a 12V power supply, the maximum current is thus limited to 20 A. For example, in SSI or UL, it is required that the highest power output by a power supply should not exceed 240VA.
There are various types of control circuits for adjusting power supply current in the market. For example, a R.O.C. patent publication No. 200619915 discloses a power supply control circuit capable of selecting current-limiting modes, wherein under an energy limitation standard, the user can utilize a selection circuit to switch the operation modes of the current-limiting/comparing circuit, including: a one-to-one mode, a one-to-multiple mode, a multiple-to-multiple mode and a multiple-to-one mode, according to different combinations of loads in a single power supply, and different power supplies with different specifications are thus unnecessary.
However, the user has to judge the power usage conditions of back-end electronic devices before he selects the mode of the current-limiting/comparing circuit via the selection circuit. If the user does not understand the power usage conditions of the back-end electronic devices, he is unable to select the mode of the current-limiting/comparing circuit correctly, and the electronic devices will thus not operate normally. Besides, as the selection circuit is a manually-operated switch in practice, an unintended touch will incur the sudden power change of the electronic devices and thus damage the electronic devices.